In an interview with the Radio Times, Steven Moffat said:
"We’re making a full series. I can confirm that. "I’m making a full series of 12 episodes, plus a Christmas special. I don’t know when it goes out. That’s up to someone else. And even if I did know – which I genuinely don’t – I wouldn’t be allowed to say so as I have absolutely no say in it whatsoever."
"[But] it’s not being reduced in size. We’re not making fewer episodes. That’s all complete bunk. I can confirm that absolutely."
Peter Capaldi also confirmed it when asked by BBC News's Lizo Mzimba: